Purification of benzoic acid and its derivatives



' No Drawing.

' monocarboxylic acids.

' lem of separating polycarboxylic" acids from Patented July '15,] 19,30

' PPerms:a;

LLo'Ynfc. DANIELS, F ma as, rmm vwmm; AssIenoa-mo'afin "snnnniv con rang, or r rr'rsnuas a, rnnnsvnvemma'coaronnr onor DELAWARE This invention relates to the separation of polycarboxylic acids from'the correspondlng Various reactions for the splitting off of carboxyl groups from polycarbox y' lic acids have been developed and as a result the probthe corresponding monocarboxylic acids'has become acute because in the ordi nary proc esses of splitting a reaction product isobftained Which contains both'the monocar- Y so boxylic acid desired and considerable quan: tities of unchanged polycarboxylic acid. -Ex-' am'ples of mixtures are phthalicand benzoic acid and halogen, nitro-," amino-:orjthioph-c thalic and benzoic acids and hydrogenated derivatives; naphthalicand'naphthoic acids I )low cost; The separation of 'male cac d from I and their substituted derivative's diph'enic and, phenylben'zoic acid maleic or:

and acrylica'cid etc.

I According f to the'fpresent in vention'the than the dissociation constant of theicorresponding monocarboxylic acid. For example, s the first dissociationl constant of phthalic acid is l.21 1 O-, as compared to 6 X 10- for benzoic acid and'even greater differences may be noted between maleic acid with a first dissociation constant Of 1.17 X'1O- Lxl f compared to acryhc 5 6 O and umanc the use of any solvent in which the salt of the 9.3'X 10" Similar conditions of varying orders of magnitude are tobenoted inthe' case of other polycarboxylic acidsand their corcarboxylic acid. Similarly, if a mixture of' salts of the two acids arelpresent anda strong mineral acid is addedto neutralize theycorrect amount of base, thiswill result again in the production ofamixtureof the;m'oI-iosalt of" appea fileiij r m 3, 1929. Serial" no, 353,577,;

carboxylic acid. V r

:Afterpartiail neutralization or acidificathe'polycarboxylic acid if:

tion the' mixturefof thesalt of the po'lycarboxylic 'a'cidfand thejfreemonocarboxylic acid can beflrreadilyp separated because the two compounds show very, different solubilities'.

In some casesthe-difi'erence injsolubility in water I is 'sufiiciently great as 'efi'ect clean-cut separation. Thisis for; example, the case with mixtures of; phthalic and benzoic acids and with mixtures of naphthalics and naph- 7' thoic acids here the; benzoic :,or naphthoic acid respectivelyj'is relatively insoluble in water. The fmonoalkali met'al'jphthalate or naphthalate isi of course; soluble inwater and a very easy separation'isthereby obtained at 7' acrylic acid,,hovvever;' ,requires a different 1mm as the solubility of' acrylic acid in wate r is too great to permit. of a cle'ancut separation. Therefore; a solvent such as ethermaybeused 1n wh ch acrylic acid-1s With benzoic acid and phthalic acid by chooszfai'rly soluble whereas themonosodium male-' Q 7 late or fumarate is practicallyinsoluble, Oi" coursea similarf procedure could be, used.

ing a solvent for benzoic acid, such as chloroform or benzol, but isnormally not to be considered because wherever the'monoc arboxylic acid-is relatively insoluble Water it is invention, however is not limited to using water as f a solvent even. in the case. of such acidswhere Water could be used but includes "polycarboxylic acid andvthe free monocar boxylic acid possesses markedly; difierent solubilities. i i a p When-water 1s used as a solvent and in some cases when organic solvents are used it shouldbe borne inmind that the solubility of fore, desirable toipurifythe mixture-0f the acids as much aspossible before subjecting them to'the present process. A purification by fractional sublimation-and thalike is ire '1nuch cheaper to use ater asfa solvent. The

A mixture containing '5 parts of benzoicacid and parts of phthalic acid, which mixture is relatively free from other impurilties, is dissolved in150c. c. of water. 5.3

parts of sodium carbonate, corresponding to 98% of the amount required to convert the phthalic acid to the primary sodium salt, are added and themixture is chilled to 7 C., the benzoic acid permitted to crystallize out and the mixture filtered off and washed with about parts of icewater. The filter cake, which weighs 4.7 parts, has a melting point of 121 122 OJ. and represents benzoic acid of high purity. The recovery is 94%.

' A mixture containing similar'amounts of naphthalic and naphthoic acids is subjected to the same treatment using, of course, the corresponding amount of sodium carbonate. Excellent recovery of naphthoic acid of high purity is obtained andthe filtrate contains monosodium naphthalate contaminated only with a very little naphthoic acid.

1E sample 2 ,A mixture of benzoic and phthalic acids such as that described .in Example'l is completelyneutralized in hot water solution by adding, 15.9 parts of sodium carbonate and then suflicient' N/ 10 sodium hydroxide to produce faint alkalinity to litmus. The solution is cooled and 10% sulfuric acid is added in an amount equivalent to 9.7 parts of 100% 7 H250 constituting 97.5% of the amount of mineral acid necessary to liberate the benzoic acid and convert the phthalate to the primary salt. The mixture is chilled and'filtered as in Example 1, the benzoic acid cake obtained being 4.55 parts, which corresponds to a recovery of 91%. 5 v

The phthalic acid can be recovered from the sodium acid phthalate by acidification and cold filtration. A recovery of 88% is obtained and the product is of excellent qualit and can be used to form further amounts 0 benzoic acid.

A similar procedure may be used in separating substituted phthalic acids from substituted benzoic acids," such as the chlorbrom-, nitro-, amino-, or halogen nitroacids, and can also be applied to the separation of naphthalic acid from naphthoic acid or diphenic acid from phenyl-benzoic acid, using in each case the correct amount of sodium j carbonate and sulfuric acid.

I Ewample 3 A mixture containing about-.1 part of acrylic acid and 3 parts of maleic acid is dis solved in water and neutralized with sutficient sodium carbonate to convert the maleic acid into the primary sodium salt. The solution is then treated with ether or other organic solvent for acrylic acid, either by a batch process or by permitting the organic solvent to flow in countercurrent as described in connection with the separation of benzoic acid and phthalic acid in thepatent to A. O. J aeger, No. 1,685,634? dated September 25, 1928.

If desired the water solution may be heated and part of the acrylic acid may be boiled off, leaving a residue which contains maleic and acrylic acids which may then be leached or washed with ether or other organic solvents for acrylic acid.

What is claimed as new is:

1. A method of separating polycarboxylic acids from the corresponding incnocarboxylic acids, which, comprises transforming the polycarboxylic acid into the primary salt while leaving the monocarboxylic acid in the free state and subjectin the mixture thus obtained to the action of a solvent in which the polycarboxylic salt and monocarboxylic .acid show unequal solubility.

. 2. A method according to claim 1, in which aflprimary alkali metal salt of the polycarboxylic acid is formed.

3. A method according to claim 1, in which a mixture of the acids issubjected to partial neutralization .by an alkali in order to form the primary salt of the polycarboxylicacid.

4. A method according to claim 1, in which the two acids are aromatic carboxylic acids.

5. Amethod of separatinga polycarboxylic acid from its. correspondingmonocarboxylic acid which does not possesshigh solubility in water, which comprises forming a primary water soluble salt of the polycarboxylic acid and separating themixture thus obtained by filtering an aqueous dispersionat a temperature at which the monocarboxylic acid is relatively insoluble in water.

6. A method according to claim 5, in which this primary salt formed is an alkali metal sa t.

comprises forming a water soluble primary 7 salt of the phthalic acid and subjecting the mixture thus formed to separation by means of water at a temperature at which the benzoic acid is not readily soluble in water.

10. A method according to claim 9, in which 7 A method according to claim 5, in which the primary salt of the phthalic acid is a monoalkali metal salt. V

11., A method according to claim 9, in I which the primary salt of the phthalic acid is formed by neutralization of a mixture of the phthalic andbenzoic acids with just'sufficient alkali to form the -monoalkali metal salt of the phthalic acid.

12. A method of separating'phthalic .acid from benzoic acid, which comprises forming a water soluble primary salt of phthalic acidand subjecting the mixture thus formed to separation by means ofwater at a 'temper ature at which benzoio acid is not readily soluble in water.

13.. A method according to claim .12, in-

which the primary salt of the phthalic acid isa monoalkali metal salt.

14. A method according to, claim 12, in which the primary salt of the phthalic acid is formed by neutralization of amixture of V the phthalic and benzoic acids with just suflicient alkali to form the monoalkali metal salt of the phthalic acid.

' Signed at Pittsburgh, Pennsylyania, this 4th day of April, 1929.

LLOYD C. DANIELS. 

